The invention relates to cutting elements for rotary drill bits of the kind used for drilling or coring holes in subsurface formations. The invention may be applied to a number of different kinds of rotary drill bits, including drag bits, roller cone bits and percussion bits.
By way of example, the invention will be primarily described in relation to cutting elements for use on rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string and an inner passage for supplying drilling fluid to the face of the bit, the bit, body carrying a plurality of cutting elements. Each cutting element comprises a preform element, often in the form of a circular tablet, including a cutting table of superhard material having a front cutting face and a rear face, the rear face of the cutting table being bonded to a substrate of material which is less hard than the superhard material.
The cutting table, which is normally in the form of a single layer, usually comprises polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the cutting table and substrate are bounded together during formation of the cutting element in a high pressure, high temperature forming press. This forming process is well known and will not be described in detail. The interface between the superhard cutting table and the substrate is usually flat and planar.
Each preform cutting element is normally mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the bit body. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as "LS bonding". The bit body itself may be machined from metal, usually steel, or may be moulded using a powder metallurgy process.
Such cutting elements are subjected to extremes of temperature and heavy loads when the drill is in use down a borehole. It is found that under drilling conditions spalling and delamination of the superhard cutting table can occur, that is to say the separation and loss of the diamond or other superhard material over the cutting surface of the table.
One feature which is believed to increase the occurrence of spalling and delamination is the fact that the superhard layer and its substrate have different material properties, such as different coefficients of expansion, elastic modulus etc., leading to high levels of stress at or near the interface between the two layers. Also, it is believed that, during drilling, shock waves may rebound from the internal planar interface between the two layers and interact destructively, leading to component failure. The combination of these effects is believed to result in spalling and delamination of the cutting table at lower energies that might otherwise be the case. Indeed, the problem is so bad on occasions that polycrystalline diamond layers have been known to delaminate spontaneously from the substrate as a result of residual stresses alone.
One method which has been employed to overcome the worst of this problem is disclosed in U.S. Pat. No. 4,784,023. According to the disclosure in this patent, there is employed a non-planar interface between a polycrystalline diamond layer and a cemented tungsten carbide substrate. Typically this takes the form of grooves ground into the surface of the carbide substrate, on to which the polycrystalline diamond layer is subsequently formed. The grooves act to increase the surface area of attachment and give a non-planar interface zone between the diamond and carbide. It is also known, although not in cutting elements of the kind last referred to, to provide one or more transitional layers between the cutting table and substrate, and these are known to improve the attachment of the cutting table to the substrate. Such transition layers are used to facilitate the production of cutters having curved geometries, such as domed cutters, which may be difficult to make without some form of transitional zone. However, hitherto the interfaces between such transition layers and the cutting table and substrate have been planar, or smoothly curved in the case of domed cutters.
The object of the present invention is to provide a new and improved preform cutting element which may overcome or reduce the spalling and delamination problems referred to above. The invention also provides methods of manufacturing such preform cutting elements which may be simpler and less costly than the manufacturing methods used hitherto.